1. Field of the Invention
The present invention is directed to reducing CO2 emissions from syngas conversion processes. In particular, the present invention is directed to use of a multi-stage Fischer-Tropsch reaction system to reduce CO2 emissions from syngas conversion processes.
2. Description of the Related Art
The conversion of natural gas assets into more valuable products, including combustible liquid fuels, is desired to more effectively utilize these natural gas assets. The conversion of natural gas to more valuable products generally involves syngas generation. Syngas generation involves converting natural gas, which is mostly methane, to synthesis or syngas gas, which is a mixture of carbon monoxide and hydrogen. Syngas may be used as a feedstock for producing a wide range of products, including combustible liquid fuels, methanol, acetic acid, dimethyl ether, oxo alcohols, and isocyanates.
There are two main approaches to convert remote natural gas assets into conventional transportation fuels and lubricants using syngas. Natural gas may be converted into syngas followed by a Fischer Tropsch process, or natural gas may be converted into syngas followed by methanol synthesis, which is followed by a methanol to gas process (MTG) to convert methanol into highly aromatic gasoline. The syngas generation is the most costly step of these processes. A critical feature of these processes is producing syngas with a desired H2/CO ratio to optimize formation of the desired products and to avoid problems in the syngas formation step.
Syngas can be generated from three major chemical reactions. The first involves steam reforming of methane. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 3.0. A second process for syngas generation involves dry reforming of methane or the reaction between carbon dioxide and methane. An attractive feature of this method is that carbon dioxide is converted into syngas; however, this method has problems with rapid carbon deposition. The carbon deposition or coke forming reaction is a separate reaction from the one that generates the syngas and occurs subsequent to the syngas formation reactor. However, the reaction of methane in dry reforming is slow enough that long residence times are required for high conversion rates and these long residence times lead to coke formation. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 1.0. A third process for syngas generation involves partial oxidation of methane using oxygen. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 2.0. However, in commercial practice, some amount of steam is typically added to a partial oxidation reformer in order to control carbon formation and the addition of steam tends to increase the H2/CO ratio above 2.0.
It is possible to produce syngas with a H2/CO ratio that is a above the ratio ideally desired for the process in which the syngas is to be used, and then to remove excess hydrogen to adjust the ratio to the desired value. However, the H2 removal process employs expensive H2 separation systems that tend to foul and decline in performance with use.
The Fischer-Tropsch and MTG processes both have advantages and disadvantages. For instance, the Fischer-Tropsch process has the advantage of forming products that are highly paraffinic. Highly paraffinic products are desirable because they exhibit excellent combustion and lubricating properties. Unfortunately, a disadvantage of the Fischer-Tropsch process is that the Fischer-Tropsch process emits relatively large amounts of CO2 during the conversion of natural gas assets into salable products. An advantage of the MTG process is that the MTG process produces highly aromatic gasoline and LPG fractions (e.g., propane and butane). However, while highly aromatic gasoline produced by the MTG process is generally suitable for use in conventional gasoline engines, highly aromatic MTG gasoline may be prone to form durene and other polymethyl aromatics having low crystallization temperatures that form solids upon standing. In addition, the MTG process is more expensive than the Fischer-Tropsch process and the products produced by the MTG process cannot be used for lubricants, diesel engine fuels or jet turbine fuels.
Multiple Fischer-Tropsch reactors have been used for various purposes. For example, U.S. Pat. No. 6,169,120 to Beer of Syntroleum, discloses a two-stage Fischer-Tropsch process that uses intermediate water removal. The process of Beer, however, is intended to be used for processing syngas produced from air that will contain appreciable amounts of nitrogen.
U.S. Pat. No. 4,279,830 to Haag et al. of Mobil, discloses a second zeolite-containing catalyst used to maintain a syngas composition so that a H2:CO ratio remains in a range of about 0.5 to about 1.0. Haag explains that this composition range is appropriate for Fe-based Fischer-Tropsch catalysts, but is not suitable for Co-based catalysts. Haag also explains that operating within a selective range of process conditions provides an improved process for upgrading the total effluent from a Fischer-Tropsch operation.
U.S. Pat. No. 4,624,968 to Kim et al. of Exxon, discloses a two-stage Fischer-Tropsch operation wherein specific catalysts are used for olefin synthesis and then conversion.
WO/0063141 to Clark et al. of Reema International Corp., discloses a Fischer-Tropsch process for synthesizing hydrocarbons that includes multiple Fischer-Tropsch reactor stages arranged in series. The process of Clark provides very low carbon monoxide conversion per Fischer-Tropsch reactor stage and employs intermediate removal of water between reactor stages. In one embodiment, the system uses an iron-based catalyst. Also, in a preferred embodiment, CO2 is recycled from the last reactor in a series of Fischer-Tropsch reactor stages to a syngas reactor.
EP 0679 620 A2, to Long of Exxon, discloses a high conversion hydrocarbon synthesis achieved by reacting H2 and CO in a first stage(s) in the presence of a non-shifting catalyst. The process further includes separating liquid products and reacting the remaining gas streams in the presence of shifting catalysts.
Additionally, the conversion of CO2 into hydrocarbonaceous products using dual functional syngas conversion catalysts has been described in various references. For instance, “Development of Composite Catalyst Made of CuZnCr Oxide/[HY]Zeolite for Hydrogenation of Carbon Dioxide,” Fujiwara M; Kieffer R; Ando H; Souma Y, Applied Catalysis A: General V 121 M.1 113–24 (Jan. 5, 1995); “Hydrocarbon Synthesis From CO2 Over Composite Catalysts,” Souma Y; Kieffer R; Fujiwara M; Ando H; Xu Q 4th International Carbon Dioxide Utilization Conference (Kyoto Japan Sep. 7–11, 1997) Studies in Surface Science and Catalysis V114 327–32 (1998); “Hydrogenation of Carbon Dioxide Over Cu—Zn—Cr/Zeolite Composite Catalysts: The Effects of Reaction Behavior of Alkenes on Hydrocarbon Synthesis,” Fujiwara M; Ando H; Tanaka M; Souma Y, Applied Catalysis A: General V130 N. 1 105–16 (Sep. 14, 1995); “Hydrogenation of Carbon Dioxide to C1–C7 Hydrocarbons by a Methanol on Composite Catalysts,” Inui T; Kitagawa K; Takeguchi T; Hagiwara T; Makino Y, Applied Catalysis A: General V94 N. 1 31–44 (Jan. 27, 1993) and “Preparation of Benzene Fractions of Hydrocarbons—Includes Using Catalysts Containing Specified Zeolite and Metal Oxide Constituent,” K G Ione and V M Mysov, Ru2089533, all disclose converting CO2 into hydrocarbonaceous products using dual functional syngas conversion catalysts.
Thus, while the use of multiple Fischer-Tropsch reactors is known for various purposes, until now no one has suggested using a multi-stage reactor system that employs selected catalysts to reduce CO2 emissions. As a result, there is a need for processes that reduce CO2 emissions from syngas conversion processes while producing desired hydrocarbonaceous products.